class_docstr: Base class for feeds.
  
  :param maxLen: The maximum number of values that each :class:`pyalgotrade.dataseries.DataSeries` will hold.
      Once a bounded length is full, when new items are added, a corresponding number of items are discarded
      from the opposite end.
  :type maxLen: int.
  
  .. note::
      This is a base class and should not be used directly.
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code: def getNextValuesAndUpdateDS(self):
      dateTime, values = self.getNextValues()
      if dateTime is not None:
          for key, value in values.items():
              # Get or create the datseries for each key.
              try:
                  ds = self.__ds[key]
              except KeyError:
                  ds = self.createDataSeries(key, self.__maxLen)
                  self.__ds[key] = ds
              ds.appendWithDateTime(dateTime, value)
      return (dateTime, values)
  
file_path: pyalgotrade/pyalgotrade/feed/__init__.py
imports: import abc
  from pyalgotrade import observer
  from pyalgotrade import dataseries
label_desc: Use image node_id 6 for calling the BaseFeed obj's underlying member method code with example usage: obj.getNextValuesAndUpdateDS() and returns: dateTime,  values